Terephthalic acid and other aromatic carboxylic acids are widely used in the manufacture of polyesters, commonly by reaction with one or more glycols, and particularly ethylene glycol and combinations thereof with one or more higher homologues of alkylene glycols, for conversion to fiber, film, containers, bottles and other packaging materials, and molded articles.
In commercial practice, aromatic carboxylic acids are commonly made by liquid phase oxidation in an aqueous acetic acid solvent of methyl-substituted benzene and naphthalene feedstocks, in which the positions of the methyl substituents correspond to the positions of carboxyl groups in the desired aromatic carboxylic acid product. Oxidation is conducted by contacting the feedstock with air or another source of oxygen, which is normally gaseous, in the presence of a catalyst comprising cobalt and manganese promoted with a source of reactive bromine as described in U.S. Pat. No. 2,833,816. The oxidation is exothermic and yields the aromatic carboxylic acid together with by-products, including partial and intermediate oxidation products of the aromatic feedstock as well as oxidation and other reaction products of the acetic acid solvent such as methanol, methyl acetate, methyl bromide, carbon monoxide and carbon dioxide. Water is also generated as a by-product. The aromatic carboxylic acid oxidation product, by-products and intermediate oxidation products of the feedstock are commonly formed dissolved or as solids suspended in the liquid phase reaction mixture and are commonly recovered by crystallization and solid-liquid separation techniques.
Pure forms of aromatic carboxylic acids are often favored for manufacture of polyesters for important applications, such as fibers, bottles, and other containers and packaging materials, because impurities, such as by-products generated from aromatic feedstocks in oxidation processes such as those described above and, more generally, various carbonyl-substituted aromatic species, are known to cause or correlate with color formation in polyesters made from the carboxylic acids and, in turn, off-color in polyester converted products. Aromatic carboxylic acids with reduced levels of impurities can be made by further oxidizing crude products from liquid phase oxidation as described above, for example at one or more progressively lower temperatures and/or oxygen levels or during crystallization steps commonly used to recover products of the oxidation, for conversion of feedstock partial oxidation products to the desired acid product, as known from U.S. Pat. Nos. 4,877,900, 4,772,748 and 4,286,101. Preferred pure forms of terephthalic acid and other aromatic carboxylic acids with lower impurities contents, such as purified terephthalic acid or “PTA”, are made by catalytically hydrogenating less pure forms of the acids, such as crude product comprising aromatic carboxylic acid and by-products generated by liquid phase oxidation of aromatic feedstock, so-called medium purity products or other impure forms of the acids, in solution at elevated temperature and pressure using a noble metal catalyst. In some commercial operations, liquid phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid and purification of the crude product are often conducted in continuous integrated processes in which crude product from liquid phase oxidation is used as starting material for purification.
A difficulty in manufacture of aromatic carboxylic acids by such processes results from use of bromine-promoted oxidation catalysts. Bromine sources used with the catalyst and reaction products thereof formed during oxidation are corrosive. To limit corrosion, process equipment, including major equipment items such as oxidation reactors and off-gas treatment equipment, is normally constructed using titanium or other expensive, corrosion-resistant metals or alloys. Treatment of process off-gases to avoid atmospheric emissions of volatile bromine compounds formed in the process, such as by thermal or catalytic oxidations to oxidize organic bromine compounds to carbon oxides and molecular bromine and reduction of the latter to anionic bromine using sodium formate, also are commonly used, adding complexity and cost to manufacturing processes.
Eliminating bromine from conventional cobalt-manganese oxidation catalysts is not practical for commercial scale aromatic carboxylic acid manufacture because yields of desired products are unacceptably low. In addition, oxidation of acetic acid solvent for the liquid phase reaction tends to increase in cobalt and manganese-catalyzed oxidations without bromine. Sacrificial promoters, such as methyl ethyl ketone and acetaldehyde, have been proposed as alternatives to bromine, as known from U.S. Pat. No. 3,361,803, but their use in practical applications is disfavored because they are consumed in oxidation, not only adding costs for replacing the consumed promoter but also diverting oxygen from desired reactions. These sacrificial promoters can also negatively affect product quality in higher temperature oxidations. N-hydroxyphthalamide has been reported in Y. Ishii, J. Mol. Catal. A.: Chem, 1997, 117 (1-3, Proceedings of the 6th International Symposium on the Activation of Dioxygen and Homogeneous Catalytic Oxidation, 1996), 123, as a bromine-free alternative promoter for cobalt-catalyzed reactions but its utility in manufacture of aromatic carboxylic acids is limited by its low solubility in acetic acid oxidation reaction solvent, and its consumption, and conversion to undesirable by-products during oxidation due to multiple competing decomposition reactions.
German Patent No. 2804158 describes a process for manufacture of terephthalic acid by solvent-free co-oxidation of p-xylene and/or p-tolualdehyde plus methyl p-toluate to dimethyl terephthalate using a bromine-free catalyst composed of cobalt or manganese salts or a combination of manganese with cobalt or with zinc salts at a temperature in the range of 140-240° C. according to the so-called Witten-Hercules process, followed by catalytic hydrogenation of the total reactor effluent from the co-oxidation in the presence of a palladium, platinum, nickel or cobalt catalyst. The process also includes a heat treatment step for transesterification of terephthalate and p-toluate mono- and diesters from either the co-oxidation or hydrogenation step, which is conducted at 180-350° C. after removal of hydrogen and volatiles. Heat treating is conducted under a nitrogen atmosphere, preferably with addition of water and methanol and, for reducing treatment time, optionally in the presence of Mo, W, Ti, Mg, Ca, Sr, Ba, Mn, Fe, Ni, Zn, Y, K, Y, La, Ce, Nd, Sm, Re, In, Sb, Bi, Se, Te, Sn, P or combinations thereof as catalyst. Absence of bromine from the oxidation catalyst and of monocarboxylic acid reaction solvent in the oxidation are said to permit use of less corrosion-resistant metals for construction of equipment for the co-oxidation and lessen solvent burning.
U.S. Pat. No. 3,865,870 describes a process for oxidizing methylated benzenes to carboxylic acids in which streams of water, methylated benzene feedstock and oxygen-containing gas are fed concurrently over a catalyst metal in a reactor pressurized to 300-1200 psi and at temperatures of 170-300° C. The catalyst metal is silver, palladium, ruthenium, platinum, rhodium, iridium or osmium and is supported on alumina, silica, titania, zirconia, silicon carbide or carbon. Para-xylene oxidations with the patent's preferred metals, ruthenium, palladium or silver, resulted in low yields of oxidized para-xylene derivatives, low aromatic carboxylic acid selectivities (e.g., 3-5% with palladium) and often with significant generation of carbon oxides due to burning of para-xylene feedstock according to the patent's examples.
U.S. Pat. No. 6,160,170 of Codignola discloses oxidation of aromatic feed materials to aromatic carboxylic acids with gaseous oxygen in the absence of bromine in a liquid phase reaction mixture including an aqueous organic solvent using a homogeneous catalytic complex characterized generally as consisting of (A) at least one Group VIIIA metal with a valence greater than 2; and/or at least one Group VIIA metal and/or cerium; and (B) and at least one Group IVA metal which is preferably zirconium or hafnium. (Groups VIIIA, VIIA and IVA referred to in the patent correspond, respectively, to Groups VIII, VIIB and IVB of more recent versions of the Periodic Table according to US Patent Application No. 2002/0188155 A1 of Codignola et al.) Catalyst compositions described in the patent consist of cerium acetate and zirconium acetate, and of ruthenium oxide and zirconium acetate. Practical effectiveness of the catalysts for manufacture of aromatic carboxylic acids is limited because water in amounts commonly present in product recovery or other process steps can rapidly convert zirconium (IV) acetate to zirconium (IV) oxide, which, due to its insolubility in water, can be difficult to separate from aromatic carboxylic acid products recovered in solid form, cause plugging of equipment and catalysts in downstream processing and diminish quality of purified aromatic carboxylic acids products. Precipitated zirconium (IV) oxide also represents a loss of catalyst metal. US Patent Application No. 2002/0188155, noting instability of the catalysts according to International Application WO 98/2938, to which U.S. Pat. No. 6,160,170 corresponds, and reduced activity and selectivity due to their degradation, proposes low temperature (90-150°) oxidation using bromine-free catalytic complexes as in the patent preferably containing a Group VIII metal or cerium and zirconium or hafnium and preferably a mixture of cobalt or cerium and zirconium salts, with filtration of the oxidation product and return of mother liquor from filtration to oxidation, all under substantially the same temperature and pressure conditions. In addition to added complexity of the process, catalysts according to this citation show strong activity for oxidation of acetic acid reaction solvent to carbon oxides unless reaction temperatures are maintained below about 120-140°.
U.S. Pat. No. 5,877,330 describes catalysts prepared from polyvanadic acid sols and other metal compounds for use in gas phase hydrocarbon conversions, reporting 99.5% conversion in high temperature gas-phase oxidation of o-xylene with air at 320° C. with 73.6% selectivity to phthalic anhydride using a calcined combination of polyvanadic acid sol and titanium dioxide and 16.1% conversion of toluene with selectivities of 22.9% to benzaldehyde and 30.1% to benzoic acid using a combination prepared from polyvanadic acid sol and boehmite.
Oxidation of selected aromatic substrates to alcohols and their esterification reactions using catalysts unpromoted with bromine are known from the following patents and publications but oxidation to aromatic carboxylic acids are not described.
Combinations of palladium and antimony are reported useful for production of benzyl mono- and bis-acetates by oxidation of toluene with oxygen gas in an acetic acid solvent to benzyl alcohol and esterification thereof by reaction with the acetic acid according to JP 10265437 A2, and by esterification of mono- or bis-hydroxy products resulting from oxidation of para-xylene in acetic acid according to JP 2004137234 A2. Oxidations did not progress beyond the benzylic alcohols in either case. U.S. Pat. Nos. 5,183,931 and 5,280,001 state generally that alkyl aromatics having a benzylic hydrogen can be oxidized to corresponding oxidized products selected from acids, aldehydes, alcohols and esters by contact in the presence of an oxygen containing fluid in a reaction medium with a catalyst composed of a palladium salt, a lithium, sodium, potassium, magnesium or calcium persulfate, an alkali or alkaline earth metal salt and a tin salt. As demonstrated in the patents' examples, all oxidations were conducted in alkaline reaction media of acetic acid with added potassium acetate and the only reactions exemplified are conversions of p-t-butyl toluene to p-t-butyl benzyl acetate. As in the Japanese publications, oxidations did not proceed beyond formation of the benzylic alcohol, which underwent esterification with acetic acid from the reaction medium.
Tanielyan, S. K. and Augustine, R. L., “Acetoxylation of Toluene Catalyzed by Supported Pd—Sn Catalyst”, J. Mol. Cat. 1994, 87, 311, reports oxygen uptake in stages corresponding to color changes and residue formation during reaction of toluene with oxygen in acetic acid solvent in the presence of palladium(II) acetate, tin(II) acetate and potassium acetate and proposes a reaction mechanism in which an homogeneous Pd/Sn complex forms and Sn(II) is oxidized to Sn(IV) by oxygen in a first stage, the Pd/Sn(II) complex absorbs oxygen and generates Pd/Sn(III) or Pd/Sn(IV) intermediates which undergo oxidation and reduction reactions to generate Pd(0)/Sn(IV) in a second stage, and the resulting Pd(0)/Sn(IV) catalyzes acetoxylation of the benzylic carbon atom in the third stage.
Other catalytic oxidations of substituted aromatic compounds using catalysts containing or prepared from palladium components and particular other metal components or combinations are reported in U.S. Pat. No. 6,245,936, U.S. Pat. No. 4,804,777 U.S. Pat. No. 6,476,258 and US 2004/0158068. The oxidations according to those patents are conducted in alkaline reaction media and/or for preparation of aryloxyacetic acids from aryloxyethanol starting materials by oxidation of the alcohol of the ring-bonded oxyethanol group but without oxidation of the carbon atoms of ring-bonded carbon-containing substituent groups that may be present in the starting materials.
Antimony (III) is known as a polycondensation catalyst for the manufacture of polyethylene terephthalate from terephthalic acid and glycols. It is considered to have sufficient Lewis acidity to catalyze the polycondensation and not to undergo oxidation, although Leuz, A-K.; Johnson, C. A., Geochemica et Cosmochimica Acta 2000, 69(5), 1165, reports that oxidation of trivalent antimony to pentavalent antimony can occur in the presence of oxygen at a pH greater than 9.8 but not at a pH range of 3.6-9.8, or in the presence of hydrogen peroxide at a pH range of 8.1 to 11.7 but not at pH below 8.1.